Project Summary Our long-term goal is to understand the biological mechanisms underlying GWAS hits in LOAD and other tauopathies. The objective of our current proposal is to develop mouse models in which the entire 190 Kb human MAPT gene precisely replaces the complete 157 Kb mouse Mapt gene, and to use these models to compare the molecular endophenotypes of tau in young and old mice expressing H2 or H1 MAPT haplotypes. Our rationale is that the non-coding variants that define the H1 and H2 haplotypes are present in MAPT but not Mapt, necessitating the incorporation of the full MAPT gene. In addition, because we have found that the location of MAPT in the mouse genome affects its expression, we will preserve the basic structural configuration of Chr17q21.31 in which MAPT resides, and maintain its relationship relative to other genes in the gene cluster that mice and humans share. These studies will be significant because they will constitute the first in vivo bioassays of non-coding polymorphisms, paving the way for future research on GWAS hits in not only LOAD, but also other chronic medical disorders. Our overarching hypothesis is that differences in the progression and pattern of tau mRNA and protein expression underlie the variations in risk associated with H2 and H1 MAPT haplotypes. We have in-hand the first MAPT Gene-Replacement (GR) mouse line, in which the 190kb human H2 MAPT precisely replaces the 157 Kb Mapt locus, and will use the same methodology to create a precisely matched line in which the H1 MAPT replaces Mapt. We will measure and compare mRNA, protein and post-translational modifications of tau in young and old GR1 and GR2 mice. Upon completion of these studies, we will have created and characterized two lines of mice in which Mapt is precisely replaced by H1 or H2 variants of MAPT. We predict that the distinct polymorphisms in H1 and H2 MAPT will suffice to alter the molecular endophenotype of tau in GR1 relative to GR2 mice, and that these differences will increase with age.